This invention relates to a method of and a facility for converting a first synchronous communications signal of a first synchronous digital communication system to a second synchronous communications signal for a second-communication system. In particular, the invention relates to a method of and a facility for converting a SONET signal to an SDH signal and vice versa.
User data to be transmitted over a synchronous digital communication network are packed in containers (C). The containers may vary in size according to the user data volume. The containers are provided with an overhead xe2x80x94this combination is referred to as a virtual container (VC)xe2x80x94and are packed in transport frames referred to as synchronous transport modules (STMs). Smaller virtual containers are first combined into a larger virtual container. The larger virtual containers can be positioned freely within the frames. A pointer in the overhead of a frame points to the beginning of the larger virtual container. The combination of the larger virtual container and the pointer is called an administrative unit (AU). In the overhead of the larger virtual container, pointers point to the beginning of the smaller virtual containers. The latter together with the respective pointers associated with them are referred to as tributary units (TU).
Two variants of synchronous digital communication systems are known. In Europe, a system known as Synchronous Digital Hierarchy (SDH) is used, while the North American System is known as the Synchronous Optical Network (SONET). The two systems differ only slightly, but as a result of the differences, the systems are not directly compatible.
In the multiplex hierarchy of SDH, the largest virtual container is the VC-4 with a capacity of 149 Mb. In the VC-4, several smaller containers of the VC-3, VC-2, or VC-12 type can be multiplexed. The VC-4 together with its pointer is called an AU-4, and the smaller containers with the associated pointers are called TU-3s and TU-2s. An STM-1 transport module carries one AU-4, which, in turn, may contain three TU-3s.
In SONET, the largest container is the VC-3with a capacity of 49 Mb. The VC-3 may contain several smaller containers of the VC-2 or VC-11 type. In SONET, the virtual containers are also frequently referred to as xe2x80x9cvirtual tributariesxe2x80x9d (VT). The VC-3 together with its pointer is called an AU-3, and the smaller containers with their associated pointers are called TU-2s. A container compatible with the VC-4 of SDH does not exist in SONET. In an STM-1 frame, also referred to in SONET as an STS-3 (electrical) or an OC-3 (optical), three AU-3s are transported, each of which may contain seven TU-2s. The SDH and SONET multiplex hierarchies described are specified in ITU-T G.707, Chapter 6. Accordingly, compared with the multiplex hierarchy of SONET, the multiplex hierarchy of SDH comprises an additional level, namely the VC-4. The other levels of the two multiplex hierarchies correspond to each other. The hierarchy levels of the two systems are shown in FIGS. 8 and 9 and are explained in detail below.
To connect a synchronous digital communication network of the SDH type with a synchronous digital communication network of the SONET type, plesiochronous interfaces, such as DS-3s or E1s, have so far been used, which support a transmission rate of 45 Mb. The user data are transmitted at the VC-3 level via the plesiochronous interface, and at the other end, new VC-3s are formed. This has the disadvantage that SDH monitoring functions, such as end-to-end monitoring of the transmission path, cannot be executed, since the overheads of the containers are not transmitted via the plesiochronous interface.
As an alternative, ITU-T G.707, Chapter 6.4, specifies a direct interconnection of the synchronous transport modules STM-1 of SDH and SONET at the VC-3 level. An AU-3 of SONET is reduced to a VC-3. For the VC-3, a new pointer is determined, thus forming a TU-3. Three TU-3s are combined into a VC-4, and a pointer is determined for the VC-4 to form an AU-4, which is then transmitted in a synchronous transport module STM-1.
In a synchronous digital communication network, alarm surveillance and performance monitoring are carried out for each virtual container. To that end, overhead bytes of the individual virtual containers are interpreted. The results are communicated to a central network management system. As the multiplex hierarchies of SDH and SONET differ, the monitoring functions to be performed differ also. Therefore, SDH and SONET systems each have their own network management system. The above-mentioned direct connection at the STM-1 level between SDH and SONET involves the difficulty that SONET-specific monitoring functions have to be performed on the SONET side of the conversion facility, and SDH-specific ones on the SDH side. Thus, the network element that performs the conversion must perform monitoring functions of both systems. This involves a considerable amount of complexity, since the network element then requires two logically separate controllers. In addition, problems may occur in the cooperation of the two logically separate controllers. It is therefore not desirable to have to perform monitoring functions of different communication systems in one network element.
It is therefore an object of the invention to provide a method whereby a communications signal of a first synchronous digital communication system can be converted to a communications signal of a second communication system without having to perform monitoring functions of both systems. Another object of the invention is to provide a facility for converting a communications signal of a first synchronous digital communication system to a communications signal of a second communication system which requires only one controller for performing monitoring functions.
These objects are attained by a method of converting a first synchronous communications signal of a first synchronous digital communication system (SONET) to a second synchronous communications signal for a second communication system (SDH), wherein the first and second communications signals are framed multiplex signals which are composed of multiplex units (TU-2, TU-3, AU-3, AU-4) according to a respective specified multiplex hierarchy in such a way that smaller multiplex units (TU-2, TU-3) are multiplexed in larger multiplex units (AU-4, AU-3) and that the larger multiplex units (AU-4, AU-3) are transmitted in frames (STM-1), wherein the multiplex hierarchy of the first communication system (SONET) comprises at least one level corresponding to a level of the multiplex hierarchy of the second communication system (SDH), whose multiplex hierarchy includes an additional level (VC-4) in comparison with the multiplex hierarchy of the first communication system (SONET), and wherein the multiplex units (AU-3) of the first communications signal are converted to corresponding multiplex units (TU-3) of the second communications signal, which are then multiplexed into multiplex units (AU-4) of the additional hierarchy level,, characterized in that monitoring functions according to the multiplex hierarchy of the second communication system (SDH) are applied to both communications signals, and that in applying the monitoring functions to the first communications signal, the additional level of the multiplex hierarchy of the second communication system (SDH) is simulated by setting parameters to be monitored to default values.
The invention further resides in a facility for performing this method.
Further features and advantages of the invention will be apparent from the detailed description below.
The invention has the advantage of permitting end-to-end monitoring at the VC-3 level.